Technical Field
The present invention relates to height measurement at small distances and, in particular, to the use of optical interference to determine absolute vertical position above a surface.
Description of the Related Art
Optical interference can provide a precise measurement of a change in distance between a lens surface and a target surface. However, interference patterns (e.g., Newton's rings) are typically periodic, repeating their pattern with height changes on the order of the wavelength of the light used. Measuring absolute distances between the lens and the target surface is therefore difficult.